1. Field of the Invention
The present invention relates to an optical recording medium comprising a substrate, and at least a recording layer and light-transparent layer formed on the substrate, and a method of manufacturing the optical recording medium. More particularly, the present invention concerns an optical recording medium having a multilayered substrate structure contributing to an improved strength and durability and also capable of recording data with a high density, and a method of manufacturing the optical recording medium.
2. Description of Related Art
Optical and magnetic recording media are widely used to record various kinds of information such as audio, video and other information. More particularly, they are generally classified into an optical disc in which information signals are previously written in the form of embossed pits, a phase-change optical disc in which information signals are written utilizing the phase change of its recording layer, a magnetooptic optic in which information signals are written utilizing the magneto-optical effect of its recording layer, and a magnetic disc in which information signals are magnetically written.
These disc-like recording media use a resin-made substrate in which phase pits, pre-grooves, and so forth are formed as data, tracking signals, and so forth in the form of delicate grooves and lands.
Conventionally, such a resin-made disc substrate is molded using an injection mold as shown in FIG. 1. FIG. 1 is a sectional view of the injection mold.
The injection mold is generally indicated with a reference 100. It consists essentially of a fixed mold 101a to form one of the main surfaces of a disc substrate, a movable mold 101b disposed opposite to the fixed mold 101a to form the other main surface of the disc substrate, and a mold 112 to form a periphery of the disc substrate. The fixed mold 101a and movable mold 101b are provided each with a stamper having formed thereon grooves and lands corresponding to a desired groove/land pattern indicative of information signals, and so forth.
The movable mold 101b is moved toward and away from the fixed mold 101a by a driving mechanism (not shown). When clamped together, the fixed mold 101a, movable mold 101b and peripheral mold 112 define together a cavity 111.
The fixed mold 101a has provided therein and positioned in the center of the cavity 111 a nozzle 114 to fill, by injection, the cavity 111 with a molten synthetic resin into the cavity.
In the injection mold 100 having the above configuration, the movable mold 101b is first moved toward the fixed mold 101a (clamping) by the driving mechanism not shown) to define the cavity 111. Next, the molten synthetic resin is filled into the cavity 111 by injection through the nozzle 114.
Then, the injected synthetic resin is cooled by a thermoregulator (not shown) to a slush state. The movable mold 101b has a first ejecting member 116 disposed therein. A punch 117 is thrust from the central hole of the ejecting member 116 toward the fixed mold 101a to make a hole which will be a center hole in a disc substrate. Thereafter, in the injection mold 100, the injected synthetic resin is hardened by cooling by the thermoregulator (not shown).
Then, in this injection mold 100, the movable mold 101b is moved away from the fixed mold 101a (mold opening) by the driving mechanism (not shown). Finally, the disc substrate formed in the cavity 111 is taken out by a stripping mechanism (not shown).
Thereafter, a recording layer, reflective layer, protective layer, and so forth for example, are formed on the resin-made disc substrate thus molded to produce an optical disc.
However, the above-mentioned injection molding used to produce an optical disc is disadvantageous in that at the step in which a molten resin is filled, by injection, in the injection mold, a change of the injecting pressure, change of the injecting temperature and a friction between the molten resin and mold will cause stresses in the molten resin in the cavity.
More particularly, such stresses will take place at the following steps. First, during injection of the molten resin into the injection mold 100, the molten resin injected in the cavity 111 will flow to cause a shear stress. Next, when the injection of the molten resin into the cavity 111 is complete, a screw (not shown) for injecting the resin abruptly stops moving and also the molten resin stops flowing abruptly. Thus their respective inertia will take place as stress. Also in the process up to gate sealing for injection of the molten resin, the molten resin is pressurized to prevent the molten resin from flowing and a sink from taking place due to a volumetric shrink of the molten resin. An uneven pressure distribution in the entire disc substrate will result in a stress. Especially when cooling to harden the molten resin, an uneven temperature distribution will take place in an outer portion 120a (see FIG. 2) of the molten resin in contact with the fixed mold 101a and movable mold 101b as well as in an inner portion 120b (also see FIG. 2) not in contact with the molds, thus causing a stress.
A part of such stresses will be partially relaxed in the process until the molten resin is cooled and hardened in the mold, but the majority will reside, not relaxed, as a residual stress in the molded disc substrate.
As a result, the molded disc substrate 120 will be subject to deformations such as partial warpage 121 and sink 122 and an uneven distribution of birefringence or double refraction as shown in FIG. 2.
The disc substrate molded from a resin by injection molding is unavoidably shrunk in the molding process, especially at the cooling step. More particularly, the shrinkage is different in the outer periphery from in the inner periphery of the disc substrate in many cases. The outer periphery of a disc substrate 130 warps, resulting in a dish-like deformation as shown in FIG. 3.
Thus, to minimize the deformation of the disc substrates 120 and 130, it is required for the conventional injection molding that mold clamping should be done with a reduced force and injection be made at a slower speed to reduce the packing rate, thereby reducing the pressure inside the resin. However, such measures taken in the conventional injection molding for manufacture of disc substrates are very troublesome but cannot practically attain any completion elimination of the deformation of the disc substrates 120 and 130.
Because of such deformation of the disc substrates 120 and 130, a predetermined land/groove pattern cannot be formed on the disc substrates 120 and 130 with a high accuracy but the disc substrate 120 incurs a poor stamping 123 of the land/groove pattern as shown in FIG. 4. As the result, the optical disc produced using such disc substrate 120 or 130 is disadvantageous in that its signal characteristic is not satisfactory.
The recording density of an optical disc depends upon a diameter of a laser spot focused on a recording layer of the optical disc. That is, the smaller the laser spot diameter, the higher the recording density is. The laser spot diameter is proportional to a product xcex/NA (xcex: laser wavelength and NA: numerical aperture) of a reading/writing optical system. For an increased recording density in an optical disc, it is necessary that a laser having a shorter wavelength xcex and an objective lens having an increased numerical aperture NA should be used.
However, such an increased NA of the objective lens will raise a problem of coma aberration because the coma is proportional to ([skew angle]xc3x97NA3xc3x97[optical disc thickness through which a laser light passes]). To cope with this coma problem, it has been proposed to reduce the thickness of the transparent substrate for optical disc.
Normally, however, there is a relationship that the strength of an optical disc is proportional to a cube of the thickness of the disc. A conventional optical disc including a substrate of which the thickness is reduced for an increased recording density is disadvantageous in that the mechanical properties such as bending strength, and so forth are inferior and a bimetallic deformation is very easily arisen due to a moisture absorption, and so forth in addition to the aforementioned stresses.
It is therefore an object of the present invention to overcome the above drawbacks of the prior art by providing an optical recording medium having a sufficient strength and durability without deformations such as warpage, sink, and so forth and capable of recording data with a high density, and a method of manufacturing the optical recording medium.
According to the present invention, there is provided an optical recording medium including a substrate, and a recording layer and transparent layer formed in this order on the substrate, a light being incident from the transparent layer to write and/or read information signal, the substrate comprising:
a first resin layer forming a surface of the substrate on which the recording layer is formed; and
a second resin layer formed on the first resin layer from a resin having a higher flexural modulus than that of the resin of the first resin layer.
In the optical recording medium having the above-mentioned structure according to the present invention, the substrate is comprised of the first resin layer forming the surface of the substrate on which the recording layer is formed, and the second resin layer formed from the resin having the higher flexural modulus, namely, a higher rigidity, than the resin of the first resin layer. Therefore, the substrate has an increased strength to minimize the deformation of the substrate. As the result, the optical recording medium according to the present invention is enhanced in strength, thus hardly subject to deformations such as warpage, sink, and so forth and has an outstanding durability. Since in this optical recording medium according to the present invention, a laser light is incident from the transparent layer side, the structure of the substrate composed of the first and second resin layers will not adversely affect the optical writing and/or reading.
Further, the first resin layer forming the surface of the substrate on which the recording layer is formed is formed from the resin having the smaller rigidity, namely, a better fluidity, than the second resin layer. Therefore, the first resin layer has an excellent stampability, and thus the substrate has a superior stampability.
According to the present invention, there is provided a method of manufacturing, by injection molding, an optical recording medium including a substrate, and a recording layer and transparent layer formed in this order on the substrate, a light being incident from the transparent layer to write and/or read information signal.
In the optical recording medium manufacturing method according to the present invention, the substrate is formed by two-color molding steps of forming a first resin layer at least on a side of the substrate on which the recording layer is formed, and a second resin layer on the first resin layer from a resin having a higher flexural modulus than the resin of the first resin layer.
In the optical recording medium manufacturing method according to the present invention, the substrate is formed by forming, by the two-color molding, the first resin layer on the surface of the substrate on which the recording layer is formed, and the second resin layer formed from the resin having the higher flexural modulus, namely, a higher rigidity, than the resin of the first resin layer. Therefore, the substrate has an increased strength to minimize the deformation of the substrate. As the result, the method according to the present invention can be used to manufacture an optical recording medium enhanced in strength, thus hardly subject to deformations such as warpage, sink, and so forth and having an outstanding durability.
Further, in the method according to the present invention, the first resin layer forming the surface of the substrate on which the recording layer is formed is formed, from the resin having the smaller rigidity, namely, a better fluidity, than the second resin layer. Therefore, the first resin layer has an excellent stampability, and thus the substrate has a superior stampability.